1. Field of the Invention
The present invention generally relates to a hybrid vehicle control device. More specifically, the present invention relates to hybrid vehicle control device for implementing engine stop control in order to maintain the efficiency of exhaust purification during restarting of the engine, the control device having a drive mode in which travel is accomplished by one or both of an engine and a motor.
2. Background Information
One example of a conventional hybrid vehicle control device is disclosed in Japanese Laid-Open Patent Publication No. 2007-64097 for a hybrid vehicle equipped with a three-way catalytic converter in the engine exhaust path for simultaneously purifying emissions in exhaust gas (HC, CO and NOx) by maintaining a theoretical air fuel ratio in a catalyst atmosphere. In this conventional hybrid vehicle control device, an engine stop control is performed for stopping the engine so that the purification efficiency of an exhaust purification catalyst is maintained during engine restarting. In this engine stop control, when the engine is stopped, the output shaft of the engine is braked by first closing the throttle valve and increasing electrical generation torque of a motor/generator while engine fuel injection is continued. When the rotation of the engine output shaft stops or reduces to a prescribed low speed, the engine fuel injection is stopped. In this technique, by controlling the output rotation with the aid of a differential allowance mechanism when the engine speed is reduced, the vehicle speed does not decrease in conjunction with the decrease in engine speed. The term “differential allowance mechanism” refers to a mechanism in which a torque generating element is connected to each rotating element of a planetary gear, such as in the configuration described in Japanese Laid-Open Patent Publication No. 2007-64097. This differential allowance mechanism has the function of allowing the engine speed to decrease while maintaining the output speed (i.e., the vehicle speed) by controlling the torque of each torque generating element. The term “differential allowance mechanism” as used in the present specification always has the same meaning, and does not, for example, include a planetary gear (differential mechanism) provided in a stepped automatic shifter.